One good way to control them if you don't have too many is to just pull them by hand. Try to get a large portion of the root with it and it will be less likely to come back in the future. It helps to use a spade to get under the plant to pry it out of the soil. They liked to encroach on our plots also, we found that mowing(we used a weedeater on some also) them just after the tall stalks start to appear will also set them back and they eventually will die.